Surgical implants requiring a filler material, such as breast implants, penile implants, or musculature implants, have conventionally used a silicone gel or a saline solution as the filler material. One of the primary concerns in the design of an implant is to provide a tactile response that is comparable to the tactile response of normal human tissue. Silicone has been the filler material of choice primarily because silicone filled implants provided an adequate tactile response, but the very nature of the silicone filler material also limits the desirability of its tactile properties. Silicone filled implants typically comprise about 10-20% cross-linked silicone which forms an interconnected "sponge" in the implant with the remainder of the filler material being low molecular weight silicone oil. Because of this "sponge", silicone filled implants are stiffer than human tissue, such as breast tissue, and do not provide the same feel as human tissue. Moreover, adverse medical consequences have recently become associated with the use of silicone gel filled implants because it has been discovered that the silicone oil can migrate through the implant shell and the silicone oil is not biocompatible with other human tissues. Therefore, the use of silicone based filler materials has been discontinued in the industry. Saline filled implants have not exhibited any adverse medical consequences when used as a filler material in an implant, but saline is not considered a preferred filler material because of the relatively low viscosity (.about.1 cp) of such a solution which renders undesirable its tactile properties.
In U.S. Pat. No. 4,995,882 issued to Destouet et al., the rights to which are owned by the assignee herein, naturally occurring, vegetable derived triglyceride oils were proposed for use as a filler material for a breast implant. This represented a dramatic break from the teaching of the prior art and is a good and valuable invention. One of the stated reasons for using a triglyceride oil composition was to take advantage of the radiolucent characteristics thereof so that a useful mammogram could be performed of an implanted breast. Although this radiolucency characteristic is important for mammograms, the naturally occurring triglyceride oil compositions disclosed by Destouet et al., such as peanut oil and sunflower seed oil, have a relatively low viscosity (.about.30 cps) and as such do not completely provide the desired tactile response of a normal breast. Furthermore, naturally occurring triglyceride oils typically contain unsaturated and/or polyunsaturated alkyl groups which are susceptible to oxidation. It is known that polyunsaturated fatty acid side chains of triglycerides will react with molecular oxygen to create unstable reduction products such as superoxide and hydrogen peroxide. This is known as lipid peroxidation or rancidification. The reaction forms peroxy lipid radicals and eventually results in the formation of shorter chain compounds such as malondialdehyde, ethane, pentane and 4-hydroxyalkenals, free fatty acids, and the unstable superoxide and peroxide moieties. The free radicals produced as a result of lipid peroxidation are capable of harming cells and cellular components as well as molecular structures essential to the organism. Because of the hydrophobic nature of lipid radicals, it is believed that membrane associated molecules would be most at risk, but other molecular species such as non-oxidized lipids, carbohydrates, lipoproteins and nucleic acids could be damaged. Thus, it is believed that the oxidation of unsaturated alkyl chains on a triglyceride may reduce over time the biocompatibility of naturally occurring triglycerides in an implant.
There is, therefore, room for improvement with a filler material which retains the radiolucent characteristics of a naturally occurring triglyceride oil while also having an improved viscosity and biocompatibility, and implants containing such a filler material.